1. Field of the Invention
The present invention relates to a receiver used in a Code Division Multiple Access (CDMA) communication system.
2. Description of the Related Art
As CDMA digital car telephone and portable telephone systems, there is known a North American standard system (TIA IS95).
In the standard specification (TIA/EIA/IS-95-A) published by the TIA(Telecommunications Industry Association), chapter 6 has a description concerning functions required for a mobile station, and chapter 7 has a description concerning functions required for a base station.
A brief outline of the North American standard system is as follows:
That is, a basic chip rate is about 1.23 Mcps, and as current accommodated user rates, there are two rates of a 14.4 kbps sequence and a 9.6 kbps sequence. The forward channel includes a pilot channel that all mobile users use commonly, and under a precondition that base stations are synchronous, seamless handover (soft handover) is achieved by acquiring pilot synchronization.
Pilot channel signals are used in estimation of radio channel and coherent detection, or measurement of a base station transmitted signal level.
Further as a signal reception system, there is adopted a RAKE combining reception system. That is, maximal-ratio combining is performed on signals corresponding to different paths received in multipath environment, whereby a received signal is obtained.
It is important in receiving a CDMA signal to perform initial synchronization acquisition and synchronization tracking processing with high rate.
In a CDMA communication, it is planned to increase a bit rate to 64 kbps to enable communications of character information and image information besides speech information. Under a condition that such high rate signal processing is required, capabilities of a synchronization tracking system in the above-mentioned conventional receiver are not necessarily sufficient.
Accordingly if a phase of a received signal rapidly shifts before tracking processing is started after the initial synchronization is established, it is expected that the worst problem may occur that already established synchronization is lost. This case causes the necessity to perform the initial synchronization acquisition processing again.
For example, under such a condition that a mobile station such as an automobile moves along a slop with a height variation with very high speed, when the soft handover is performed, there is expected a case that a phase of a received signal rapidly shits due to switching between base stations, and thereby the synchronization is lost before the tracking processing starts working.
The use of a digital signal processor (DSP) is considered to speed up the tracking processing. In this case, however, power consumption is increased because software loop processing is repeated many times. Further it is considered that capabilities for the other processing in the DSP be decreased by an amount corresponding to an additional program for reception processing.
The present invention is carried out based on the consideration as described above. It is an object of the present invention to reduce the time taken to start the synchronization tracking processing without increasing loads on software and without increasing power consumption, and to speed up the tracking control.
The reason for the long time taken to start tracking processing in a conventional receiver is that a pilot filter to remove a noise from a pilot signal is present in a control loop.
In the present invention, the pilot filter is arranged externally of a path (loop) to perform the synchronization tracking processing. That is, the pilot filter is arranged independently to enable the filtering processing of a pilot symbol and tracking control processing to be performed concurrently.
According to this constitution, it is possible to speed up the processing required to determine a throw amount (a shift amount in reception timing) because it is not necessary to perform the filtering processing of the pilot signal, and consequently to perform smooth synchronization tracking control. Further this constitution does not cause such problems that a load on software is increased and power consumption is increased.
In one aspect of a CDMA receiver of the present invention, at least one pilot filter, which performs filtering on a pilot symbol transmitted from a base station to decrease an interference component, is provided externally of a signal processing loop for synchronization tracking. It is thereby possible to concurrently process the filtering of the pilot symbol and signal processing for the synchronization tracking.
Since the throw amount is determined without waiting for the processing with the pilot filter to be finished, the time taken from signal reception to start tracking control is reduced. Thereby, in performing a soft handover, such an inconvenience hardly occurs that once established synchronization is lost.
Further in another aspect of the CDMA receiver of the present invention, data indicative of the throw amount of a reception timing is generated from a table (look-up table system) when the synchronization tracking is performed while shifting the reception timing per a predetermined interval basis. Then a relationship between inputs and outputs in the table is updated using synchronization establishment information transmitted from the base station.
In the IS-95 CDMA communication system, the forward channel includes a think channel and handover assist channel besides the pilot channel and traffic channel. A mobile station side always receives synchronization establishment information (useful information for synchronization establishment) transmitted from the base, station. Accordingly when the soft handover is performed, the mobile station side knows phase information of a pilot channel signal of the base station to which the communication is switched. The use of the information achieves faster convergence of a tracked phase as compared to a case that tracking control is started under a condition of no preliminarily knowledge. Accordingly tracking control performance is further improved.
Furthermore, in the other aspect of the CDMA receiver of the present invention, a time constant of the pilot filter is updated using information concerning automatic frequency control.
A pilot signal component varies due to carrier frequency offset of a local oscillator at a receiver side, frequency shift caused by fading, and other facts. A range that the pilot signal component is considered to be in (frequency band) is estimated using the information from an automatic frequency control circuit (AFC circuit), and the time constant of the pilot filter (width of a passing band) is changed dynamically. That is, at the time of the soft handover, since a variable parameter value in the AFC circuit is changed, according to the change, the time constant of the pilot is also changed.
It is thereby possible to decrease noise components (interference components) effectively to detect the pilot signal accurately, and therefor to improve a received quality in obtaining a demodulated signal from a received signal. As a result, it is possible to prevent the deterioration of the received quality in performing the soft handover.